Wash quinoa in 3 changes of cold water in a bowl, draining in a sieve between changes of water. Stir together quinoa, 4 cups water, and 3/4 teaspoon salt in a 2- to 3-quart saucepan, and simmer, uncovered, until quinoa is just tender and germ starts to separate from grain, about 20 minutes. Drain well in sieve, then transfer to a medium bowl. While quinoa is simmering, cover bulgur with warm water by 2 inches and soak until tender and chewy, about 10 minutes. Drain well in a sieve, then stir into drained quinoa. Cool grains completely, about 20 minutes. While grains cool, stir together oil, lemon juice, mint, pepper, and remaining 3/4 teaspoon salt in a small bowl and let stand 15 minutes, then stir into grains along with olives, radishes, feta, and lettuce. Serve immediately.